* Debris pattern could indicate midair breakup
* Brazil defense minister says explosion unlikely
* Air France confirms recent bomb threat on a flight (Adds defense minister comments)
By Miguel Lo Bianco
FERNANDO DE NORONHA, Brazil, June 3 (Reuters) - Search crews flying over the Atlantic found debris from a crashed Air France jet spread over more than 55 miles (90 km) of ocean on Wednesday, reinforcing the possibility it broke up in the air.
But Brazilian Defense Minister Nelson Jobim said the existence of large fuel stains in the water likely ruled out an explosion, undercutting speculation about a bomb attack.
"The existence of oil stains could exclude the possibility of a fire or explosion," he said at a news conference in Brasilia. "If we have oil stains, it means it wasn't burned."
Experts said extreme turbulence or decompression may have caused the Airbus A330 to splinter two days ago on its way from Rio de Janeiro to Paris with 228 people on board.
The first Brazilian navy ship was nearing the crash area, about 685 miles (1,100 km) northeast of Brazil's coast, to begin retrieving debris. French officials said they may never discover why the plane went down as the flight data and voice recorders may be lost at the bottom of the ocean.
Air force pilots searching the area have reported no signs of survivors and officials said recovering bodies may be extremely difficult.
"As well as bodies sinking, you also have problems along the coast of Pernambuco (state) that you know about," Jobim said in reference to sharks. He added bodies could take several days to float to the surface.
Newly spotted traces of the plane included a 12-mile (20-km) fuel stain and various objects spread across a 3-mile (5-km) area, including one metallic object 23 feet (7 metres) in diameter.
The plane sent no mayday signals before crashing, only automatic messages indicating electrical faults and a loss of pressure shortly after it entered stormy weather.
"If the decompression reading was correct, it caused a structural problem ... it is a very violent event that causes pieces to come apart and that explains why the wreckage is spread out so much," said Kirk Koenig, a commercial pilot and president of Indianapolis-based Expert Aviation Consulting.
"It's like when you see an Indy 500 race car being hit and pieces start to come off," he added.
Aviation trade publications focused on a series of warnings in recent months issued by U.S. and European regulators about electronic systems on A330s and A340s that could throw planes into sharp dives. The directives covered ADIRUs -- air data inertial reference units -- which feed crucial information to the cockpit to help fly planes.
With officials struggling to explain how a modern aircraft could have crashed in stormy weather that is routine on the trans-Atlantic route, there was speculation a bomb could have caused the worst crash in Air France's 75-year history.
The airline said on Wednesday it had received an anonymous telephone warning that a bomb was on a flight leaving Buenos Aires on May 27, four days before the crash. A spokesman said the plane was checked, no bomb was found and the aircraft left an hour and a half late. He added that such alerts were relatively common.
Given the challenging location of the crash, its cause may never be known.
"I am not totally optimistic. We cannot rule out that we will not find the flight recorders," said Paul Louis Arslanian, the head of France's air accident investigation agency.
MINI-SUB ON ITS WAY
France is dispatching a mini-submarine that can explore to a depth of 19,680 feet (6,000 metres) and will try to locate the Airbus' flight data and voice recorders, which should shed light on the crash.
The recorders are designed to send homing signals for up to 30 days when they hit water, but there is no guarantee they even survived the impact with the sea, Arslanian said.
Brazil is leading its search effort from Fernando de Noronha, a sparsely populated volcanic archipelago and nature reserve off its northeastern coast.
It has mobilized 11 air force planes, four navy vessels with divers and a tanker for the retrieval operation that Jobim said was being carried out in a 120-mile (193-km) radius.
Jorge Amaral, a Brazilian air force colonel, said the long strip of metal found on Wednesday was the biggest piece that search crews had seen so far.
"We are considering this 7-metre piece to be part of the plane, possibly part of the side, a piece of steel. It could be part of the fuselage or the tail," he told reporters.
The French investigation will have its first report ready by the end of the month, and will be led by Alain Bouillard, who took charge of the investigation into the crash of an Air France Concorde in 2000.
France held an ecumenical religious ceremony for relatives and friends of those on the plane at Notre Dame Cathedral in Paris on Wednesday, attended by President Nicolas Sarkozy. (Additional reporting by Alonso Soto and Rodrigo Viga Gaier in Rio, Laure Bretton and Clement Guillou in Paris; Writing by Stuart Grudgings; editing by Terry Wade and Peter Cooney)